Does this performance degradation only occur when the "Full Data View" is open and selected from the 'View Data' capability of Tableau Desktop?
Hope this helps!
Behavior and performance is the same in desktop.
One workaround I would consider is to create a view of the underlying data that you wish to have created as another sheet. Then use the map as a filter to filter the sheet down.
I would also be curious if viewing the underlying data for the data source within the workhseet does that also create an extensively long query time?
This sounds like a problem on Oracle's side. You probably need to compare the plans generated by Oracle and see why the one with rownum is so slow.
As for workarounds, you may be able to lie to Tableau that your Oracle doesn't support rownum or similar using connection capabilities (see Tableau Capability Customizations Reference). But I suspect that will cause other problems.